I don’t think this can be done, at least not straightforward. The recipe builder assumes the current board (the board you want are adding the integration to). You can try to make your own dropdown list and populate that with items from board#2 (custom field type) but it will be a challenge to make board#2 a configurable item.
As an alternative (not really monday style though :)) you can build a board view that grabs the configuration of your recipe and transfer those configuration values in a database.
Sorry for our delay in handling this! I agree with @basdebruin here - I don’t think this is something that can be done right now. When using the recipe builder it uses the current board as context (sending the current board as the board you want to integrate with).